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giftopnm - convert a GIF file into a portable anymap
giftopnm
[--alphaout={alpha-filename,-}] [-verbose] [-comments] [-image N] [GIFfile]
This
is a graphics format converter from the GIF format to the PNM (i.e. PBM,
PGM, or PPM) format.
If the image contains only black and maximally bright
white, the output is PBM. If the image contains more than those two colors,
but only grays, the output is PGM. If the image contains other colors,
the output is PPM.
If you have an animated GIF file, you can extract individual
frames from it with gifsicle and then convert those using giftopnm.
A GIF image contains rectangular pixels. They all have the same aspect
ratio, but may not be square (it's actually quite unusual for them not to
be square, but it could happen). The pixels of a Netpbm image are always
square. Because of the engineering complexity to do otherwise, giftopnm
converts a GIF image to a Netpbm image pixel-for-pixel. This means if the
GIF pixels are not square, the Netpbm output image has the wrong aspect
ratio. In this case, giftopnm issues an informational message telling
you to run pnmscale to correct the output.
- --alphaout=alpha-filename
- giftopnm creates a PGM (portable graymap) file containing the alpha channel
values in the input image. If the input image doesn't contain an alpha channel,
the alpha-filename file contains all zero (transparent) alpha values. If
you don't specify --alphaout, giftopnm does not generate an alpha file, and
if the input image has an alpha channel, giftopnm simply discards it.
If
you specify - as the filename, giftopnm writes the alpha output to Standard
Output and discards the image.
See pnmcomp(1)
for one way to use the alpha
output file.
- -verbose
- Produce verbose output about the GIF file input.
- -comments
- Only output GIF89 comment fields.
- -image N
- Output the specified gif image
from the input GIF archive (where N is '1', '2', '20'...). Normally there is only
one image per file, so this option is not needed.
All flags can be abbreviated
to their shortest unique prefix.
This does not correctly handle
the Plain Text Extension of the GIF89 standard, since I did not have any
example input files containing them.
ppmtogif(1)
, ppmcolormask(1)
,
pnmcomp(1)
, gifsicle(1)
<http://www.lcdf.org/gifsicle
>, ppm(5)
.
Copyright
(c) 1993 by David Koblas (koblas@netcom.com)
If you use giftopnm,
you are using a patent on the LZW compression method which is owned by
Unisys, and in all probability you do not have a license from Unisys to
do so. Unisys typically asks $5000 for a license for trivial use of the
patent. Unisys has never enforced the patent against trivial users, and
has made statements that it is much less concerned about people using the
patent for decompression (which is what giftopnm does than for compression.
The patent expires in 2003.
Rumor has it that IBM also owns a patent covering
giftopnm.
A replacement for the GIF format that does not require any patents
to use is the PNG format.
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